This invention pertains to the control of the cutting depth for continuous excavators of the general type shown in Satterwhite U.S. Pat. Nos. 3,896,571 or 3,974,580 or any continuous excavator having excavating means mounted on a structural support at the leading end of the machine. The excavating means has two or more sections which are mounted on either side of extended frame members, making it wider than the undercarriage of the excavator. Such machines have the capability of passing through a trench under excavation and advancing along its bottom so that the bottom of the cut is not visible to the operator. Closely following the excavating means on the main frame is a separately mounted moldboard/skid plate assembly. The entire machine is supported on a crawler track or rubber tired undercarriage which can be raised or lowered relative to the digging wheel to adjust its cutting depth. The moldboard blade breaks up uncut material left between the excavating means sections and scrapes the bottom of the cut clean, crowding excess materials forward. The excavating means, which works in an undercutting manner, takes these materials, along with the freshly dug material, to be discharged onto a conveyor.
Mining, and most particularly open pit mining such as for coal, typically finds the product in stratified deposits separated by "parting materials" such as clay or shale. The product can be mined in situ and loaded by a continuous excavator if contamination of the product by parting material can be minimized while mining. The parting materials may be removed by the same excavator if it can be done with minimal loss of product. Both operations have been controlled heretofore by regulation of the digging depth according to the color of the excavated material being discharged. This method is approximate at best and demands close attention by the operator.
It is notable that the parting materials in general have a lower resistivity than lignite and a higher resistivity than anthracite coals. Generally, but not necessarily, parting materials are also harder, having a higher compressive strength than either lignite or coal. The hardness, brittleness and abrasive properties of each material in combination produce a distinctive bit vibration and sound as the formation is penetrated. Most significantly, as we dig through the strata, all of these properties change with each material change. The resistivity characteristic is widely used for wireline logging of boreholes to determine the thickness and content of strata for mine evaluation and planning.
The resistivity and compressive strength values shown below may vary for the cited materials, and materials other than these may be present in a given mine however, every material encountered will have a characteristic value.
______________________________________ TABLE OF TYPICAL PROPERTY VALUES FOR VARIOUS MINE MATERIALS COMPRESSIVE STR. RESISTIVITY MATERIAL lb./sq. in. ohms/sq. cm/cm ______________________________________ lignite 800 400,000 anthracite 3,000 100 shale 6,000 5,000 combustible 4,000 1,500 shale sandstone 13,000 80,000 clay 100 1,500 marl 250 50,000 siltstone 7,500 20,000 limestone 8,000 40,000 ______________________________________
In the present invention, either tabulated property of the mined product and parting materials may be selected as a control index, comparing the measured values of the in situ material contacted by the probes to the known values for the strata. Other properties, such as dielectric strength, vibration or sound may be used as a control variable, but resistivity and compressive strength are readily measured. Resistivity, in particular, can be related directly to logging data.
An object of the present invention is to sense the location of the strata boundary of mined product and parting material relative to the cutting plane in a reliable and durable manner while excavating. A second object is to aquire this sense of the strata boundary location in a form usable for accurate control of the cutting depth of a continuous excavator.
In the present invention, probes penetrating the virgin formation enable direct measurement of material properties for the purpose of sensing stratum boundries. Copending Bryan patent application No. 07/522,467 teaches the use of a moldboard/skid plate assembly which is inherently positioned to follow the digging depth of a forwardly mounted excavating means, facilitating the use of the moldboard as a reference location for mounting such probes. Thus, the contact velocity of the probes is the forward travel rate of the excavator.
The previously mentioned Satterwhite type excavators, with their ability to travel on the floor of the trench, leave standing on the floor an undisturbed portion of the formation which passes between the digging wheel sections. This allows moldboard mounted probes to be positioned to penetrate virgin material that is above as well as below the nominal floor level.